The Internet Engineering Task Force (IETF) Internet Draft (I-D) draft-wijnands-bier-architecture-00 (which is hereby incorporated by reference) describes Bit Index Explicit Replication (BIER) as an architecture where all intended multicast receivers are encoded as a bit in a bitmask (BM), i.e., each receiver is represented by a unique bit in the BM. A router that receives a packet with the bitmask encapsulated in the header will forward the packet based on the setting of the BM towards the receiver(s), following a pre-computed tree for each of the bits in the BM.
Due to the restricted number of bits a header can carry and the scalability concerns that this raises, BIER supports the concept of receiver sets. Each packet carries a BIER set, wherein each set is identified by a unique set identifier. Usage of several of those sets can be understood as either spanning a single BIER tree encompassing all receivers in all BIER sets (linear extension of a single distribution transport) or as multiple independent sets of BIER sets (each of those being an independent multicast distribution transport).
Once the number of receivers becomes large (i.e., many sets are present) or receivers participate in many independent BIER trees (i.e., a receiver can be assigned many independent bits in many sets), assignment of a unique BM position (BMP) to a receiver is a non-trivial problem that requires an automated solution. The usual trade-offs are either a centralized (server) approach or a distributed approach which provides, at the cost of additional protocol complexity, higher scalability. Additionally, it is necessary to ensure that all receivers sharing sets must be able to support a type of tree being used to distribute the packet and the BM length of each set.